Method and apparatus for adjusting the threshold of a CMOS radiation-measuring circuit

ABSTRACT

A radiation measuring technique includes adjusting a threshold level of a radiation sensor in a radiation-measuring circuit and obtaining an output signal based on radiation dose sensed by the radiation sensor.

BACKGROUND

This invention relates to a CMOS radiation-measuring circuit with avariable threshold.

A complementary-symmetry MOSFET (CMOS) radiation-measuring circuit mayinclude transistors configured to provide a digital output that changesfrom one state to another when the radiation dose absorbed by thecircuit exceeds a threshold. The size of the devices used in the CMOSradiation-measuring circuit determines the radiation dosage that willcause the digital output of the CMOS radiation-measuring circuit tochange from one digital state to another digital state. Hence, thedesign of a CMOS radiation-measuring circuit requires carefulconsideration of the size of transistors in the circuit.

When the voltage applied between the gate and source terminals of aMOSFET exceeds a certain voltage, the MOSFET turns on. This voltage isreferred to as the threshold voltage. The threshold voltages ofp-channel type MOSFET (PMOS) and n-channel type MOSFET (nMOS) devicesare sensitive to ionizing radiation. Generally, the threshold voltage ofan irradiated MOSFET device shifts due to an increase of trapped chargein the oxide and interface states.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a system that includes a radiation sensor.

FIG. 2 is a schematic of a radiation sensor with a variable threshold.

DETAILED DESCRIPTION

As shown in FIG. 1, a CMOS radiation-measuring circuit 1 with a variablethreshold may be exposed to a source of radiation 2 to determine when athreshold radiation dose has been reached. The radiation-measuringcircuit 1 includes a radiation sensor 3 and an auxiliary circuit 4. Theradiation-measuring circuit 1 may be used, for example, as a built-inradiation-measuring circuit in standard circuits to prevent malfunctiondue to radiation hazards. The CMOS radiation-measuring circuit with avariable threshold may have a wide range of applications in spacecraftdesign, medicine, nuclear plant and personal dosimetry. Theradiation-measuring circuit may be used as or incorporated in adosimeter. Depending on its specific application, dose ranges frommillirads to tens of megarads may be detected by the circuit.

FIG. 2 illustrates details of a CMOS radiation-measuring circuit thatmay be used to measure a variable radiation dose. A pMOS transistor P1serves as the radiation sensor 3. This transistor is coupled to theauxiliary circuit 4 that includes a current mirror with pMOS transistorsP2 and P3. The pMOS current mirror generates a current that is amultiple of the reference current source (I_(sens)). For example, in oneimplementation, the current through the second side of the currentmirror (transistor P3) is ten times the reference current (I_(sens))through the first side of the current mirror (transistor P2). The sourceterminal of the radiation-sensing transistor P1 is coupled to a firstside of the current mirror (transistor P2) through a resistive elementN1. Thus, the radiation-sensing transistor P1 is biased through theresistive element N1 and the transistor P2.

The resistive element N1 causes the voltage at the drain terminal of thetransistor P2 to differ from the voltage (V_(a)) at the source terminalof the radiation-sensing transistor P1. The resistance of the element N1should not be affected by radiation. In some implementations, theresistive element N1 may include a resistor. However, the use of atransistor as the resistive element N1 can provide better efficiency andbetter control because it uses less silicon area. In one embodiment, theresistive element N1 includes an nMOS transistor with its gate terminalshorted to its drain terminal. The transistor P2 that forms the firstside of the current mirror may have its gate terminal shorted to itsdrain terminal. The source terminal of the sensing transistor P1 also iscoupled to an output load nMOS transistor N2 that is coupled to thesecond side of the current mirror (transistor P3).

The width (W) and length (L) of a channel region of a pMOS transistoraffects its behavior. The current through the left-hand side of thecircuit (I_(sens)) may be approximated by:

I _(sens) =μpC _(ox)(W/L)(V _(a) −|V _(tp1)|)²

where up is the hole mobility, C_(ox) is the gate oxide capacitance, andV_(tP1) is the threshold voltage of transistor P1. The dimensions of thedevices used in the radiation-measuring circuit can be selected so thatwhen the circuit is not exposed to radiation and the input voltage(V_(in)) across the body terminal and the source terminal of theradiation-sensing transistor P1 is zero, the gate-source voltage of theradiation-sensing transistor P1 is slightly beyond its non-irradiatedthreshold voltage value. Therefore, even when the radiation-measuringcircuit is not exposed to radiation, the radiation-sensing transistor P1is enabled.

Additionally, when the circuit is not exposed to radiation, the currentI_(sens) is magnified by the current mirror (transistors P2 and P3). Thecircuit is designed so that the voltage (V_(a)) is above the thresholdvoltage of transistor P1 (V_(tP1)). That voltage, which appears at thegate terminal of the output load transistor N2, is not large enough tosufficiently turn on the output load transistor. In summary, when theCMOS radiation-measuring circuit is not exposed to radiation, the secondside of the current mirror (transistor P3) is on, the output loadtransistor N2 is almost turned off, and the current that flows throughthe second side of the current mirror is much larger than I_(sens). Thevalue of the output voltage (V_(out)) is pulled to the supply voltageV_(dd) through the transistor P3. In that mode of operation, the outputvoltage (V_(out)) represents a high digital state.

However, when the circuit is irradiated, the absolute value of thethreshold voltage for the radiation-sensing transistor P1 increases, asdescribed above. Once the threshold voltage increases above the value ofthe voltage (V_(a)), the radiation-sensing transistor P1 turns off. Ascan be seen from the approximation for I_(sens) in the above equation,when P1 is turned off, the current (I_(sens)) through the first side ofthe current mirror (transistor P2) will drop to zero. As a result, thecurrent that flows into the output load transistor N2 also decreases. Atthis point, the voltage (V_(a)) is only slightly less than the supplyvoltage V_(dd) as a result of the voltage drop across the resistiveelement N1 and the transistor P2. This voltage (V_(a)), which alsoappears at the gate terminal of the output load transistor N2, causesthe output load transistor N2 to turn on. The output voltage (V_(out))of the CMOS radiation-measuring circuit is, thus, coupled to groundthrough the output load transistor N2. In that mode of operation, theoutput voltage (V_(out)) represents a digital low state. Therefore, theradiation-measuring circuit provides a digital output.

To allow the radiation-measuring circuit to sense different radiationdoses, the threshold voltage of the radiation-sensing transistor P1 canbe modified by changing the body bias of the transistor through theinput voltage (V_(in)). The body bias is the voltage between the bodyterminal and the source terminal of the transistor V_(bs). The voltageused to adjust the body bias may be applied, for example, from anexternal source that allows a user to adjust the value of V_(in) whenthe CMOS radiation-measuring circuit is exposed to radiation.Alternatively, the voltage used to adjust the body bias may be anintegrated voltage source based on bandgap or charge pump circuits thata user may adjust using software.

As discussed earlier, changing the threshold voltage of theradiation-sensing transistor P1 changes the dose required to turn offthat transistor. Therefore, the dose required to change the output ofthe CMOS radiation-measuring circuit from one digital state to anothercan be altered by changing the body bias of the radiation-sensingtransistor P1. A single radiation-measuring circuit can, therefore, beused to measure different dose values that the circuit may be exposedto.

The auxiliary circuit to which the radiation-sensing transistor P1 iscoupled preferably is composed of components that, as a whole, areinsensitive to threshold voltage shift due to radiation exposure.Preferably, pMOS transistors are used in the current mirror because thisconfiguration tends to be less sensitive to threshold voltage shiftsprovided the threshold voltages of each pMOS transistor shift by thesame amount. Resistive element N1 and the output load transistor N2preferably are nMOS transistors because the absolute value of thethreshold voltage shift in an nMOS transistor remains substantiallyunchanged. More specifically, the charge trapped in the oxide ispositive for both nMOS and pMOS devices, whereas the interface statecharge is positive for a pMOS and negative for an nMOS. Because thesecharge components have the same sign in a pMOS device, the thresholdvoltage of the pMOS device is shifted toward a more negative value. Incontrast, because these charge components have opposite signs in an nMOSdevice, they induce a competing effect in threshold voltage shift andthe resulting sign of the threshold voltage shift in an nMOS devicedepends on which charge component's contribution is dominant. Therefore,the absolute value of the threshold voltage shift is usually much higherin PMOS transistors than nMOS transistors.

An advantage of the foregoing techniques is the radiation dose that maybe measured by the radiation-measuring circuit 1 need not be fixed.Therefore, CMOS radiation-measuring circuits need not incorporateseveral radiation-measuring circuits to measure different radiationdoses. Also, the auxiliary circuit, used in the radiation-measuringcircuit can be relatively impervious to radiation-induced changes.

Various modifications may be made. For example, the radiation-measuringcircuit can include an nMOS current mirror, an nMOS radiation-sensingtransistor, a pMOS resistive element coupled to the radiation-sensingtransistor and a pMOS output load. Other implementations are within thescope of the following claims.

What is claimed is:
 1. An apparatus comprising: a radiation sensor witha threshold voltage; an adjustable voltage source coupled to theradiation sensor to change a threshold level of the radiation sensor; acurrent mirror; and an output load including a gate terminal and a drainterminal, wherein the radiation sensor has a source terminal coupled toa first side of the current mirror through a resistive element andcoupled to the gate terminal of the output load and the drain terminalof the output load coupled to a second side of the current mirror. 2.The apparatus of claim 1 wherein the radiation sensor forms part of aradiation-measuring circuit and the adjustable voltage source isintegrated into the radiation-measuring circuit.
 3. The apparatus ofclaim 1 wherein a state of the radiation sensor controls a state of theoutput load.
 4. The apparatus of claim 1 wherein the resistive elementincludes a transistor.
 5. The apparatus of claim 1 wherein the resistiveelement includes a resistor.
 6. The apparatus of claim 1 wherein thecurrent mirror includes pMOS transistors and each of the radiationsensor, the resistive element and the output load includes an nMOStransistor.
 7. The apparatus of claim 1 wherein the current mirrorincludes nMOS transistors and each of the radiation sensor, theresistive element, and the output load includes a pMOS transistor.